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March of Dimes-Funded Animal Experiments: An Overview

EXTENT, COST, and TYPES of ANIMALS USED

The March of Dimes does not report the number of animals used per year. The March of Dimes uses many different species of animals, including primates, cats, dogs, rabbits, pigs, hamsters, ferrets, guinea pigs, sheep, and birds.
A yearly list of approved grants is available from the March of Dimes. This list gives only a brief description of the experiment, and it is often difficult to tell from that if any particular experiment involves animals. After reviewing the grant lists for 2002, we examined the experiments that unquestionably employed vertebrate animals. The cost of these studies totaled more than $10.25 million for 70 experiments. The average cost of the animal experiments tallied was $147,000. Next, we reviewed the same grant list for the same year, this time looking for experiments that most likely involved animals but did not specifically list the name of an animal in the title or description. Including this new tally, the total amount of animal research for 2002 included 177 experiments, costing approximately $27 million. This may or may not include experiments involving tissues removed from live animals.

Our estimates reveal that the portion of experiments funded by the March of Dimes that involve animals, when compared against the total number of research grants funded, is probably about 50 percent. If one were to conservatively estimate that 50 animals were used in each grant, over 8,000 animals are involved in March of Dimes-funded research each year.

EXAMPLES of ANIMAL EXPERIMENTS FUNDED by the MARCH of DIMES

Substance Testing Experiments

The March of Dimes has conducted animal tests of many substances, including alcohol, cocaine, nicotine, benzodiazepines, retinoids, benzene, and arsenite.

Sex-Related Spatial Learning Differences After Prenatal Cocaine Exposure in the Young Adult Rat (Levin)
This study examined the effect of prenatal cocaine exposure on the learning performance of rats. Pregnant rats were given daily injections of cocaine. The study found that fetal cocaine exposure impaired the spatial learning of female rats, whereas the males showed no impairment of performance. In contrast to this study, a previous study by other authors showed prenatal cocaine exposure to result in males who were impaired and females who were unaffected in spatial learning.

Prenatal Nicotine Exposure and Cognitive Performance in Rats (Levin)
This study examined the cognitive effects (performance in a maze) on the offspring of pregnant rats given prenatal nicotine. Pregnant rats were given nicotine infusions by pumps that were implanted on the backs of the rats. The study found that the rats exhibited subtle effects of prenatal nicotine exposure on cognitive function. This effect on the cognitive performance was magnified when drugs that affect the nicotine or adrenergic system were given. The authors point out that the chronic infusion used in this study to infuse nicotine does not replicate the human situation because it does not have the repeated stet of nicotine that results from repeated cigarette smoking in humans, nor does it have the diurnal variation of high nicotine levels during the day and lower levels at night.

Fetal Nicotine Exposure Ablates the Ability of Postnatal Nicotine Challenge to Release Norepinephrine from Rat Brain Regions (Siedler)
In this study, nicotine infusions were given via minipumps implanted on the back to pregnant rats throughout most of their pregnancy to study the effect this has on the fetuses. At 30 days of age, the offspring are killed to examine the norepinephrine levels in their brains. The study concluded that the prenatal exposure to nicotine produces a deficit in noradrenergic responsiveness, a deficit which may participate in behavioral and neuroendocrine abnormalities.

Effects of Postnatal Exposure to Alcohol on Reproductive Physiology and Sexually Dimorphic Behavior in a Marsupial, the Gray Short-Tailed Possum (Fadem)
This study looked at the effects of postnatal exposure to alcohol on reproductive physiology and sexually dimorphic behavior and anatomy in adult possums. (Possums are born at an immature stage. Therefore, the researchers theorize that giving alcohol postnatally to these animals would be like giving it prenatally to animals who are born at a more mature state.) All members of the litter were injected with alcohol after birth. Some pups were killed by decapitation one hour after the first injection. Others were injected with alcohol several more times after birth and then had their gonads removed at 32 weeks. They then had subcutaneous testosterone implanted. Later these implants were removed and subcutaneous estrogen was implanted. (Six possums died during the course of these procedures.) The researchers concluded that behavior was masculinized in females and demasculinized in males given alcohol as newborns.

Prenatal Exposure to Benzodiazepine: I. Prenatal Exposure to Lorazepam in Mice Alters Open-Field Activity and GABA A Receptor Function (Chesley)
This study looked at the effect of prenatal exposure to lorazepam. Osmotic pumps that delivered lorazepam were implanted subcutaneously in pregnant mice. The authors note that results of previous animal experiments studying the prenatal binding of benzodiazepine in animals had been conflicting and had been limited by study designs, which limited the conclusions that could be drawn. The authors also note that there is “a substantial literature in animals exposed to benzodiazepines prenatally.” The authors found increased activity of the pups at three weeks of age, but, at six weeks, there was no difference when compared with controls. They found a decrease in GABA A receptor function in mature offspring.

Prenatal Benzodiazepam Administration: II. Lorazepam Exposure Is Associated with Decreased in [35 S] TBSP Binding but Not Benzodiazepam Binding (Miller)
This study looked at binding at two sites (benzodiazepam and TBSP) on the GABA receptor in mature offspring of pregnant mice treated with lorazepam. Pregnant rats were given lorazepam by osmotic pumps during part of the pregnancy. Researchers found no differences between controls and treated mice in benzodiazepam receptor binding in vivo or in vitro. They did find decreased TBSP receptor binding. The authors note discrepancies in these results from results of previous studies that they felt may be attributable to species differences, choice or route of benzodiazepam, route and interval of administration, and rearing of offspring.

Percutaneous Retinoid Absorption and Embryotoxicity (Willhite)
This study looked at the plasma concentrations and teratogenic potential of dermal applied retinoids. Retinoid samples were applied to the shaved skin of hamsters, and subsequent blood measurements were made. Pregnant animals were also given dermal retinoids and were killed before delivery. The study concluded that it is more important to compare the retinoid systemic values (adsorbed dose) than it is to compare the topical (applied dose) when interpreting the results of conventional teratogenicity bioassays. The authors write that their data confirm the observations gathered in studies in pregnant rats and rabbits that retinoid skin contact can cause teratogenicity. The authors note that the pharmacokinetic parameter for all-transretinoic acids can vary depending upon the species, dose, formulation, and route of administration. (Tested species have included dogs, hamsters, rats, and mice.) They also note that humans are 10 to 100 times more sensitive than rodents to oral retinoid-induced developmental toxicity.

Visual Development Experiments

The March of Dimes has been funding a series of “basic research” experiments dating back at least one decade to study the development of visual pathways from the retina to endpoints in the brain. Various species have been used. About $225,000 has been appropriated to animal experiments in this area in a three-year period in the late 1990s.

Expression of a Surface-Associated Antigen on Y-cells in the Cat Lateral Geniculate Nucleus Is Regulated by Visual Experience (Sur)
This study examined the presence of an antigen on cells in an area of the brain that carries visual pathways (lateral geniculate nucleus, LGN) in visually deprived kittens and cats. Unaltered cats, kittens, cats who had had one eye sutured shut for at least one year, and cats reared in the dark from birth to four to six months of age were examined. Researchers found decreased levels of this antigen in the LGN consequent to neonatal, but not adult, visual deprivation. The authors note that there have been many previous studies that have described other changes in the physiology and anatomy of cells in the LGN during normal development and following visual deprivation. Other biochemical assays have shown changes with visual deprivation.

Modification of Retinal Ganglion Cell Axon Morphology by Prenatal Infusion of Tetrodoxin (Sretavan)
This study was done to examine the mechanisms by which retinal axons achieve their precise branching patterns. Experimental cat fetuses were delivered under anesthesia, and catheters were placed through the skull. Osmotic pumps were then attached to the back of each fetus to infuse tetrodoxin continuously into the region above the optic chiasm. (Tetrodoxin abolishes nerve conduction.) Fetuses were then returned to the uterus. After birth, the retinal axons were filled with dye. The cats were killed. Researchers found that treatment with tetrodoxin prevents the normal pattern of nerve arborization. They therefore concluded that nerve development is likely brought about by a nerve activity-dependent process.

The Morphology of Retinogeniculate X- and Y-Cell Axonal Arbors in Dark-Reared Cats (Garraghty)
This study looked at the development of X- and Y-cells of the lateral geniculate nucleus in dark-reared cats. A dark-rearing chamber was used, although no descriptive details of this chamber were given. After three to five months in the chamber, the cats were killed to allow anatomic studies. The results showed that the morphological development of the X- and Y-cells was not affected by dark-rearing. However, there was a “functional loss” of LGN Y-cells, i.e., the Y-cells were present but not active. The authors cite studies dating back many years on cats whose eyes had been sutured and studies in other dark-reared cats. The authors note that the results of this study using dark-reared cats differ from the results of studies using cats reared with monocular lid suturing and cats reared with monocular enucleation paired with lid suture of the remaining eye. They could not explain the differences.

Alternative Visual Pathway Experiments

There have been several experiments conducted on hamsters and ferrets to investigate alternative visual pathways in the brain that develop if the original visual pathways are surgically destroyed.

Induction of Functional Retinal Projections to the Somatosensory System (Frost)

Development of Anomalous Retinal Projections of Nonvisual Thalamic Nuclei in Syrian Hamsters: A Qualitative Study (Frost)

Several of the experiments listed above were reviewed by Nedim Buyukmihci, V.M.D., a veterinary ophthalmologist and professor of veterinary surgery at the University of California. He writes, “The differences between the non-human animals who were used and humans may be sufficient to render whatever was learned meaningless with respect to human beings. For example, cats and ferrets are altricial animals with respect to development of their retinal and the projections to the brain (that is much development occurs after birth). In humans, much of the development has occurred prior to birth, in an environment considerably different from that in the animals. . . .Cats or ferrets are often chosen because of previous extensive work already done on these species.”

In these papers, there is no mention of possible clinical relevance of these studies. While it is not possible to predict the future with certainty, one must ask what the likelihood is of this research being able to prevent or to treat a birth defect or any other type of problem. In reviewing animal experiments dealing with amblyopia (loss of vision in an eye due to disuse of that eye) that were conducted by suturing closed one eye in kittens to study visual cortex sensitivity, ophthalmologists Stephen Kaufman, Deanna Macek, and Marvin Kraushar concluded that animal experimentation had not led to increased understanding or treatment of this disorder and that future benefits of this line of experimentation were doubtful.

Cross-Species Transplant Experiments

The March of Dimes has funded many experiments on cross-species transplants, typically dealing with methods to control rejection. As far as we can tell, these experiments have not involved primate-to-human transplants. Two March of Dimes experimenters who frequently conduct experiments in this area have written that using non-human primates as donors has been abandoned by most investigators because of the possible transmission of lethal viruses, the limited availability of primates, and the various ethical concerns related to using primates in this manner.

These March of Dimes-funded researchers are interested in investigating the use of pigs as organ donors. They do, however, use primates as recipients in their experiments.

They also used two other models to investigate rejection. One is the guinea pig-to-rat model, and the second is an in vitro model of pig endothelial cells incubated with human sera. (It is believed that the endothelium is the primary target of the rejection response.)

Several possible methods to decrease rejection have been investigated, including using cobra venom to decrease complement activity, antirejection drugs, splenectomy, and plasmapheresis.

Few primates survived the transplant experiments for more than three days, with several dying in a few hours. In one study, three control primates lived for 1-1/2 hours, compared with 68 hours in the experimental baboon. In another experimental study, one baboon survived for 17.5 days, a second died of a pulmonary embolism at eight days, and two others died at four and 24 hours due to “technical complications related to the operative procedure.” In a third experiment, one monkey died on the first day due to “hemodynamic problems,” and the other had to be killed on the eighth day “due to wound infection.”

The papers published on these experiments also discuss the species differences in the rejection process. Three of the papers reviewed follow:

Mechanism of Complement Activation in the Hyperacute Rejection of Porcine Organs Transplanted into Primate Recipients (Dalmasso)

Prolongation of Cardiac Xenograft Survival by Depletion of Complement (Leventhal)

Physiology Experiments

Many experiments dealing with physiology have been conducted by the March of Dimes in different animal models. A common statement in many of the research papers written on these experiments is that similar experiments carried out in different species have yielded conflicting results. Another problem stated by the experimenters themselves is that the animal model used may not mimic the human situation. For instance, experiments conducted on hearts removed from guinea pigs and rabbits and lungs removed from newborn pigs and rabbits may not be relevant to the living human condition.

Regulation of Developmentally Restricted Critical Periods During Vocal Learning (Schmidt; unpublished grant descriptions)
In this study, the author proposed to manipulate the nerve fibers of zebra finches to determine neural processes during song development. The stated purpose of the experiments was to gain insight into the way in which young (human) infants develop normal language skills. It is not clear what sort of birth defect this study was aimed at addressing. Additionally, it would be more applicable to rely on the mountains of human data compiled by developmental psychologists and to build a base of knowledge of human language development by studying human children.

Altering Secondary Brain Injury Through Modification of Antioxidant Response (Natale; unpublished grant descriptions)The authors attempted to mimic human neonatal brain injury by inducing brain injury in mice. This experiment is part of a 5-year, multidisciplinary research and training program that will use hundreds of mice, both juvenile and adult, aimed at defining the molecular basis for age-dependent response to brain injury and subsequent neuronal cell death. The authors then proposed using genetically modified mice with increased expression of antioxidant proteins to see if those mice were better able to survive such cortical injuries. In vitro analysis already showed, however, that these antioxidants were suppressed during cell death, and other proposed experiments are also testing treatments that have been proven effective in vitro.

Effects of Vibration Frequency and Tissue Thickness on Intrauterine Sound Levels in Sheep (Richards)
This is one of several March of Dimes-funded experiments to study intrauterine sound levels in sheep.
This study investigated the intrauterine sound pressure levels when vibratory stimuli were applied to the abdominal wall of pregnant ewes. Near-term pregnant sheep were killed, and the fetuses were removed. A hydrophone was placed into the uterus, and the uterus was then refilled with saline. Different vibration frequencies were then applied to the abdominal wall. The authors note that the experimental design used “in which the fetus was removed, may result in different absolute intrauterine sound levels compared with those of a live fetus and an intact pregnancy.” They add that investigations in humans need to be conducted. When we contacted a practicing OB-GYN physician, she said of this device that “it is rarely used today because there are better methods of determining fetal well-being.”

Changes in Work Rate to Oxygen Consumption Ratio during Hypoxia and Ischemia in Immature Rabbit Hearts (Matherne)
This study was conducted to evaluate the relative response of myocardial efficiency to reduced oxygen supply (hypoxia and ischemia) in hearts that had been removed from immature and mature rabbits. The experiment demonstrated that, in this model, a reduction in oxygen supply by hypoxia or hypoperfusion decreased efficiency in immature hearts, but increased efficiency in mature hearts. The authors note that the results in this study differ from other studies with piglet hearts where underperfusion did not change efficiency. The authors say that the conflict in these results may be due to experimental design differences or to species differences.

Responses to Converting-Enzyme Inhibition and Hemorrhage in Newborn Lambs and Adult Sheep (Rose)
This study compared the cardiovascular and hormonal responses to angiotensin converting enzyme inhibition and hemorrhage in unanesthetized newborn sheep and adult sheep. The sheep were killed at the end of the study. Overall, the researchers noted that hemorrhage had a more potent effect on blood pressure and cardiac output in newborn sheep than it did in adult sheep. Conversely, the authors note that another study reported that hemorrhage had less of an effect on newborn kittens and rabbits than in older cats and rabbits. The authors suggest that the presence of surgical stress and anesthesia may explain the differences between the studies. This study also found that the fall in cardiac output after hemorrhage was greater with converting-enzyme inhibitors in lambs, but this was not the case in the adult sheep. The data suggested that the renin-angiotensin system plays a more important role in the maintenance of cardiovascular homeostasis in newborn lambs than it does in adult sheep. The authors note conflicting results in other studies using different species (cats, dogs, rats, lambs) to study hormonal responses to hemorrhage. They suggest that the disparities may be due to different drugs, anesthesia, and/or species differences.

Effects of Cochlear Ablation on Local Cerebral Glucose Utilization in Fetal Sheep (Abrams)
This study sought to look at glucose utilization and, by implication, normal growth and maturation in the brains of near-term fetal sheep in whom bilateral cochlear ablation had been done. (The authors note that this phenomenon had already been well documented postnatally in animals.) The tympanic membrane, ossicles, and cochlea were destroyed in fetal sheep in utero. Five to eight days later, the adult sheep and, therefore, the fetuses were killed and the fetal brains examined. The authors found a decreased glucose utilization in the central auditory nuclei as well as in virtually all areas of the brain. The authors state that they do not know how to account for the finding of decreased glucose utilization diffusely in the brain, but suggest that the removal of the cochleae could result in a diffuse inactivation of the CNS. This, they say, may be due to removal of an intact auditory system, but may also be due to generalized trauma from the surgery. The authors conclude, “Whatever the cause of the lowered cerebral metabolic rate, it may have some consequences for normal maturation of the fetal brain.”

Pulmonary Venous Pressure Increase During Alveolar Hypoxia in Isolated Lungs of Newborn Pigs (Fike)
This is one of several studies conducted by the same authors and funded by the March of Dimes to look at pulmonary pressures in different species.

The purpose of this study was to determine whether pulmonary venous pressure increases during alveolar hypoxia in lungs of newborn pigs. The authors note that it has been known for more than 40 years that alveolar hypoxia causes an increase in pulmonary vascular resistance; however, the type and size of pulmonary vessels that constrict in response to hypoxia remain controversial. The question of where the constriction occurs had already been addressed in other species. The authors write, “The purpose of this study was, therefore, to determine whether pulmonary venous pressure increases in response to hypoxia in lungs of newborns of another species.” The authors say that they chose the newborn pig because “pigs are commonly used to study the pulmonary circulation and because they have been shown to have an intense pulmonary vasoconstrictor response to hypoxia.” (There is no mention of choosing a model based on any supposed relevance to the human condition.) This experiment studied lungs that had been removed from newborn pigs. In this model they found that, in piglets, hypoxia caused an increase in both the pressure in pulmonary arteries and the pressure in small venules. The authors go on to note the many species variations that have been found, as well as the many limitations of their isolate lung model as a model for the in vivo system. They note that the results of this study are similar to those obtained with newborn lambs and ferrets, but differ from results with newborn rabbits where the increase in pressure response to hypoxia was confined to pulmonary arteries only. They conclude that the site of hypoxia vasoconstriction (arterial and/or venous) in newborn lungs is species specific.

Fetal Diaphragmatic Hernia: Pathophysiology, Natural History, and Outcome (Harrison)
The March of Dimes funded a series of experiments on this topic. These studies were done to predict if correction of a congenital diaphragmatic hernia (CDH) before birth can allow lung development to resume to enable survival after birth. It is clear that the important advances in this area were made from clinical research rather than from animal experimentation.

CDH is a defect that occurs in the developing human fetus in utero and consists of abdominal contents, such as the stomach, intestines, spleen, and/or liver, moving through a defect in the diaphragm into the left side of the chest of the developing infant, preventing normal development of the lung. This results in the death of 80 to 90 percent of severely affected infants.

The natural history of CDH in the human fetus was established by sonographic studies of unborn babies afflicted with this condition, as well as by clinical studies. Examination of the lungs of babies with CDH who died shortly after birth demonstrated the pathologic findings caused by this disorder. In those children who had only a mild degree of CDH that could be operatively corrected after birth, follow-up radiologic and pulmonary function studies demonstrated that if the pressure on the lungs caused by the hernia is repaired, then the lungs can begin to repair themselves.

Experiments on fetal lambs were also conducted at the San Francisco center. There were several variations of the experiments. In one model, a rubber balloon is inflated in the left chest of the lamb to simulate a diaphragmatic hernia. (This study was very similar to an experiment that had been conducted several years previously at Johns Hopkins University School of Medicine. This prior study also experimented on sheep implanted with a balloon in the chest to simulate a CDH.) No new discoveries about CDH were made from this lamb model. Rather, what the investigators found was that their lamb model showed changes similar to those seen previously, both clinically and at autopsy, in humans with CDH. In a later variation of the experiment, the balloon is deflated at a certain point during the pregnancy to simulate surgical correction of the CDH. This improved survival of the fetal sheep. Deflating the balloon confirmed the clinical finding that the less pressure that the hernia inflicts on the lung or the shorter the time that it exists, the greater the chance of lung recovery after the pressure is removed.

In another model, the surgeons simulated the CDH by making a hole in the diaphragm of the fetal lamb to allow viscera to herniate into the chest. They then repaired the defect primarily using procedures that were being used in similar surgeries performed postnatally on children with this defect.

Attempts to repair the surgically created CDH in lambs did not fully prepare the surgeons for the skills needed to correct the defect in humans with this disorder because of the many differences in CDH in human fetuses compared to the artificially created CDH in lambs. One such difference was the presence of the liver in the herniated organs in the human babies. Five of the first six human babies died either intraoperatively or of operative complications. According to the surgeons themselves, initial attempts at in utero repair of the CDH in infants doomed to die without the surgery taught them many important technical lessons, including choice of anesthesia, choice of uterine incision, methods to reduce the hernia, need for continuing monitoring of the fetus, and postoperative control of labor.

Although many operations on fetal lambs were conducted, these served primarily to confirm the information already available from clinical studies and to give the surgeons some operative experience. The degree to which this operative experience helped the surgeons is difficult to assess, however. Despite all of this experience, the surgeons encountered difficulties in the first human surgeries that were not predicted from the preceding animal surgeries.

Fetal Surgery in the Primate: III. Maternal Outcome after Fetal Surgery (Adzick)
This experiment looked at maternal safety in a series of in utero fetal surgeries that were performed on 94 pregnant monkeys. There were different types of fetal surgery, including 46 cases where either a urinary tract obstruction or hydrocephalus was created in the fetus as part of other experimental protocols. Another 26 involved chronically chair-restrained mothers with electrodes implanted into the uterus to monitor uterine activity in response to drugs and different surgical procedures. Monkeys were “conditioned” to restraining chairs in a routine of five days of chair restraint followed by two days of caging. In the experiments designed to look at uterine activity, there was a fetal death rate of 89 percent which the researchers partly attributed to the “profound combined insult of maternal restraint, fetal surgery, and chronic catheterization of the uterus,” effects previously documented by other researchers. The experimenters note that the drug regimens to prevent uterine contractions may have been ineffective because of the magnitude of these stimuli. (It should be noted that although the papers describing these individual experiments did not mention funding by the March of Dimes, these experiments were part of the March of Dimes-supported research paper, Fetal Surgery in the Primate III, which looked at the maternal outcome of these experiments.)

This current paper, Fetal Surgery in the Primate III, examined maternal complications after fetal surgical procedures done on 94 pregnant monkeys. This paper did receive funding from the March of Dimes. The authors concluded that, although serious maternal complications occurred, many are avoidable. There were three maternal deaths, five uterine ruptures, and five cases of wound infection or dehiscence. One “anesthesia-related death occurred during a case in which there was no designated anesthesiologist.” The authors add, “Another maternal death secondary to eclampsia was also avoidable, since vigilant monitoring of maternal blood pressure was not performed after the immediate post-operative period. Of note, monkeys are quite susceptible to both eclampsia and abruptio placenta.” There were five cases of uterine rupture, three of which—including one that resulted in maternal death—were due to “technical failure of a prototype absorbable staple device.” The two uterine ruptures not due to technical failure of the staple device occurred during term labor. The authors concluded “cesarean section delivery should be mandatory after fetal surgery to avoid rupture of the relatively fresh uterine wound.” (This would not surprise any obstetrician!) All animals had been obtained from the California Primate Research Center, Davis, California.

Miscellaneous

Murine Knockout Model of 4-hydroxybutyric Aciduria (SSADH Deficiency) (Gibson; unpublished grant description)
The author proposed to “study a mouse model of SSADH deficiency, which in humans causes psychomotor retardation, hypotonia, and speech delay.” This is a funded continuation of a previous grant, in which the researchers developed a mouse model of a human inborn metabolic error that results in a buildup of the chemical gamma-hydroxybutyric acid (GHB). The mice are subject to repetitive seizures and death by 20 days of age because of their researcher-induced genetic defect, in addition to the experimental protocols they must endure. It is also not certain that the same genes control this enzyme system in humans and mice or whether by “knocking out” this gene in the mice, scientists have not deleted other biochemical characteristics that will affect any proposed treatments. Clinical research with SSADH-deficient patients and GHB users is warranted.

Identification of Genes Causing Learning and Memory Deficits in a Mouse Model of de122q11 Syndrome
For this grant, the researcher has developed a mouse “model” of a human condition known as DiGeorge/Velocardiofacial Syndrome. This syndrome is caused by genetically inherited deletions in one section of a chromosome. The author proposes to use the mouse “model” to find deletions in the murine genome related to “schizophrenia-related behavior” and psychiatric problems seen in some of them. Even though studies are being conducted elsewhere using techniques to study the human genome, the funds provided for this grant have been and will be used to generate a “panel of mouse mutants,” or mouse recombinants, to scan the genome for the gene deletions related to this murine behavioral disorder. This breeding program is likely creating hundreds of mouse varieties with different genetic modifications.

Early Differentiation of the Gonads in the Gray Short-Tailed Opossum (Monodeophis domestica) (Fadem)
This study examined the time course for gonadal development in gray short-tailed opossums. The mother opossums were anesthetized, and the neonates were removed from the teats and decapitated. The study discussed the similarities and differences of gonadal development in the opossum as compared with other marsupial species.